(678f) Dynamic Simulation of Aging in a Hard-Sphere Colloidal Glass after Volume-Fraction Jumps

In this work we develop a computational framework for studying glassy dynamics in colloidal glass-formers from a micromechanical perspective. Many molecular liquids transition from a liquid to a crystalline state upon cooling. If the cooling is sufficiently rapid, however, the material can undergo a glass transition, whereby it solidifies without crystallization, forming a glass in which the amorphous structure is retained but the dynamics become extremely slow (Angell, 1995). Similar behavior has been observed in colloidal fluids, whereby they form a colloidal glass upon a â??concentration quenchâ? (Pusey and van Megen, 1986). Much analysis in the colloidal-glass literature assumes that the behavior of colloidal systems at concentrations near the jamming transition is similar to that of a molecular glass near its glass transition, hence making the simpler colloid a good model for glass-like materials. Recent experimental studies by McKenna and co-workers (Di et al, 2011; Di, Peng, and McKenna, 2014) reveal that non-equilibrium aging and structural recovery of colloidal systems differ qualitatively from molecular glassy dynamics when interrogated for the McKenna-Kovacs signatures (Kovacs, 1963): the intrinsic isotherm, the asymmetry of approach to equilibrium, and the memory effect. The reasons for such differences have not yet been elucidated beyond the broad hypothesis that colloids may not be â??trueâ? glass-forming systems despite many shared dynamical and mechanical signatures. To bridge this gap, we study via large-scale dynamic simulation the structural relaxation, dynamics and rheology of a hard-sphere colloidal glass after various volume-fraction jumps. In this talk, we focus on the computational aspects of executing such jumps and the impact of quenching methods on the post-jump particle dynamics. Three methods are utilized to model a volume-fraction jump: container shrinkage (similar to droplet evaporation); fluid removal through a sink (mimicking fluid aspiration); and a jump in particle size (to model thermo-reversible particle swelling). During and following each jump, the positions, velocities, and particle-phase stress are tracked and utilized to characterize relaxation time scales, heterogeneous pressure, and structural changes. In addition, we implemented the protocols of the McKenna-Kovacs signature experiments to study the approach to the intransient state. For all such tests, the quench (concentration jump) was executed via each of the three methods, and the results compared to provide insight not just into implications of computational test protocol but also the effects of boundaries and quench rate on system behavior. A comparison to the creep experiments of soft colloids (Peng and McKenna, 2016) show qualitative agreement.